Pseudomonas aeruginosa is a ubiquitous and important opportunistic pathogen that possesses an impressive arsenal of virulence factors. Elastase, a metalloprotease encoded by the lasB gene, contributes to virulence by degrading or inactivating tissue and immune system components. The expression of lasB requires both the las quorum-sensing (QS) system and PQS. Little is known about the role elastase plays in the life of this bacterium. Evidence that lasB mutant cells are severely deficient in both twitching and swarming motility was recently discovered in our lab and is presented here for the first time. Surface associated motility is essential for survival and dispersal of bacterial populations and plays a defining role in biofilm formation. The proposed research will investigate the mechanism(s) by which elastase affects these two forms of motility. Twitching motility is dependent upon PilT mediated retraction of the Type IV pilus. Pilus retraction is inhibited in lasB mutant cells, thus our studies will focus on the mechanism by which LasB affects pilus retraction. Swarming motility requires both flagella and rhamnolipids. The number and position of flagella are altered on lasB mutant cells. Additionally, lesser amounts of C4-homoserine lactone (HSL) are produced by lasB mutant cells. The production of rhamnolipids, pyocyanin, and pyoverdin are all controlled by the rhl quorum-sensing system for which C4-HSL is the cognate signal. The lasB gene itself is regulated by QS and the proposed research seeks to determine how elastase affects AHL production in P. aeruginosa. The pleiotropic effects of the lasB mutation are best addressed by a global approach involving microarray and proteomic analyses which are proposed. The results of the proposed research will lead to a greater understanding of how LasB, an important virulence factor that is crucial in host tissue exploitation, also contributes to surface-associated motility, biofilm formation and QS regulation in this bacterium.